Time-lapse seismic surveying is increasingly used for studying of earth formations. It is applied for monitoring of hydrocarbon bearing underground reservoirs, in particular to follow the effects resulting from producing reservoir fluids (e.g. oil, gas, water) through a well to surface.
In time-lapse seismic surveying, seismic data are acquired at least two points in time. Time is therefore an additional parameter with regard to conventional seismic surveying. This allows studying the changes in seismic properties of the subsurface as a function of time due to, for example, spatial and temporal variation in fluid saturation, pressure and temperature. Time-lapse seismic surveying is also referred to as 4-dimensional (or 4D) seismics, wherein time between acquisitions represents a fourth data dimension. Like in conventional seismic surveying, the three other dimensions relate to the spatial characteristics of the earth formation, two being horizontal length dimensions, and the third relating to depth in the earth formation, which can be represented by a length coordinate, or by a time coordinate such as the two-way travel time of a seismic wave from surface to a certain depth and back.
The acquisition and initial processing of the seismic data can be done by standard seismic techniques. The time span between the first and the second point in time at which seismic data are acquired can be several years. One normally tries to acquire the first and second seismic data sets in a similar way, so that they are best comparable. If that is not fully possible, differences in acquisition can be accounted for during processing.
Seismic surveying techniques investigate the earth formation by generating seismic waves in the earth formation, and measuring the time the waves need to travel between one or more seismic sources and one or more seismic receivers. The travel time of a seismic wave is dependent on the length of the path traversed, and the velocity of the wave along the path.
A general difficulty in seismic surveying of oil or gas fields is that the reservoir region normally lies several hundreds of meters up to several thousands of meters below the earth's surface, but the thickness of the reservoir region or layer is comparatively small, i.e. typically only several meters or tens of meters. Resolution of processed seismic data in the reservoir region is therefore an issue. Resolution requirements are even higher when small differences in time-lapse seismic surveys are to be detected and interpreted.
The paper “4D constrained depth conversion for reservoir compaction estimation: Application to Ekofisk Field” by J. Guilbot and B. Smith, The Leading Edge, March 2002, p. 302-308, discloses a method for interpreting a time-lapse seismic survey of a subsea earth formation, in order to determine reservoir compaction as a result of production and water flooding. Seismic data were acquired at a first and a second point in time, separated by about 10 years. The seismic data were interpreted at the hand of a model of the earth formation, which consisted of, consecutively from bottom to top, a lower reservoir layer, an upper reservoir layer, an overburden layer and a seawater layer. The model included the depth of the boundaries between adjacent layers and the seismic velocity in each of the layers. It was found that in order to interpret the compaction of the reservoir correctly, it is required to take into account the changes in the seismic velocity in the various earth layers between the first and second points in time.
Among the interesting questions to be answered in a time-lapse seismic survey of a producing reservoir is about inhomogeneous depletion of the reservoir during production, caused e.g. by the presence of a discontinuity or fault that seals a certain part of the reservoir from those parts which are in direct fluid communication with the production well. Whereas the resolution of the processed seismic data is often just sufficient to obtain a more or less clear indication of the presence of a discontinuity or fault, differences observed in the reservoir region in a time-lapse seismic survey (e.g. amplitude or interval velocity changes) are generally so small that it proves to be very difficult to draw reliable conclusions about the sealing nature of the fault.
There is a need for a method of interpreting time-lapse seismic data, which allows one to obtain more detailed information about certain parameters and conditions of the reservoir region, and it is an object of the present invention to provide such a method.